The present invention relates to Fast Frequency Hopping Spread Spectrum (FFHSS) Code Division Multiple Access (CDMA) communications. More particularly, the invention relates to the transmission and reception signal processing methods and devices. The invented method avoids the requirement of fast frequency hopping synthesis in the FFHSS transmitter and receiver previously used In mobile radio communications or the like. Preferred embodiment of the invented method Is particularly suitable for fiber optical implementation of the FFHSS-CDMA technique.
Code Division Multiple Access (CDMA) communications is a technique presently used in wireless applications. CDMA accommodates a large pool of subscribers, while providing dynamic simultaneous access to an arbitrary subset of them. In a typical CDMA network, a number of K users simultaneously communicate sharing the same communication medium. This is achieved by assigning a unique code to each individual user. The assigned codes are selected so as to minimize the interference or the cross-talk between users and to reduce the synchronization loop complexity in the receiver.
In the fields of satellite and mobile communications, spread spectrum (SS) signals served as a basis of the development of CDMA network systems. SS techniques are very popular in a wide variety of fields such as satellite communications, mobile communications, naval and avionics communication systems, distance or range measurement, high resolution target and direction finding systems. There are two categories of SS systems: direct sequence (DS) system, in which each information bit is multiplied by a temporal pseudo-random sequence, and a frequency hopping (FH) system, in which the carrier frequency of a narrow-band information transmitted signal is switched (or hopped) at a random and discrete method. Slow frequency hopping (SFH) means that only one frequency-hop is achieved per bit, however, fast frequency hopping (FFH) means that a number of frequency hops are achieved for every information bit.
In a conventional FFHSS transmitter, as shown in FIG. 1, the data modulated signal is multiplied by the output of the frequency hopping synthesizer 104 using the first multiplier 1021. The frequency synthesizer 104 output signal is a wide band time periodic deterministic signal with time period equal to the duration of a one data bit modulated signal (Tb). In the following, it is assumed that only two kinds of information will be transmitted 1 and 0; FIG. 3A shows a sequence of four bits; 1010. Each bit period Tb is divided into an integer number (M) of time intervals Tc=Tb/M called chips. During every chip interval no more than one discrete frequency (or frequency band) from an available set of M frequencies (or frequency bands) is used in the frequency synthesizer 104 output signal. The M available frequencies are assigned to the M chip intervals as prescribed by the selected code from the code generator 1052FIG. 3D shows an example where the integer M is equal to 5, hence the code is composed from 5 frequencies f1, f2, f3, f4 and f5. The order of frequencies In the selected code is f3, f1, f4, f2 and f5; which means that the frequency f3 is transmitted during the first chip interval, f1 is transmitted during the second chip interval, . . . , and the last frequency f5 is transmitted during the fifth chip interval. The modulation operation using the first multiplier 1021 spreads the data modulated signal energy over a bandwidth, called spread spectrum bandwidth Wss=M*Wb, which is M times larger than the data modulated signal bandwidth Wb. Hence, the FFHSS encoding operation cuts the modulated signal energy in time into M pieces, and shifts the frequency band of each piece by an amount corresponding to the FFH code. The frequency domain of the first multiplier 1021 output signal Is usually referred as the intermediate frequency. The second modulation, achieved using multiplier 1022, shifts all the SS signal to the carrier frequency fixed by the oscillator 103. The multiplier 1022 output signal is fed to the emitting or transmitting antenna 106.
In the conventional FFHSS receiver, as shown in FIG. 2, a receiving antenna 151 provides the received FFHSS signal. A local oscillator 153 generates a signal for shifting a frequency band of a received signal to a band of an intermediate frequency. A multiplier 1521 multiplies the received signal by the local oscillator 153 output signal for shifting the frequency band to the base band domain if the synchronization is well established between the received frequencies and the locally generated frequencies. The band pass filter (BPF) 161 limits the band of the output signal of the first multiplier 1521 to the SS bandwidth Wss=M*Wb. A hopping synthesizer 154 outputs an SS signal similar to the transmitter hopping synthesizer 104 corresponding to the selected FFH code. The second multiplier 1522 multiplies the BPF 161 output signal with the hopping synthesizer 154 output signal, the product inputs the low pass filter LPF 162 which limits the band of the output signal of the second multiplier 1522 to the original data modulated signal bandwidth Wb. A power measuring device 157 measures a detection power for a one bit portion from the low pass filter 162; on the basis of this power measurement, the hopping sequence phase control equipment 166 controls the hopping synthesizer via a code generator 155 to continuously shift its hopping sequence until full synchronization is established between the received signal hopping sequence and the locally generated hopping sequence. A decision circuit 158 receives the output of the low pass filter 162 and decides about the received output.
FIG. 3 illustrates the signal evolution through the various major signal processing steps in the prior art FFHSS transmitter and receiver. FIG. 3A depicts a sequence of 4 data bits (1010) in the logical state. FIG. 3B shows the data modulated signal at the first multiplier 1021 input. In FFHSS systems, frequency shift keying (FSK) and phase shift keying (PSK) are the most popular modulation techniques in mobile radio communications. Amplitude shift keying (ASK) is less robust in wireless communications. Since only two types of information are considered, 1 and 0, only the binary cases of the modulation schemes, (binary ASK, FSK and PSK), are considered. FIG. 3C shows the time (Tb) versus frequency bandwidth (Wb) allocated to the data modulated signal in the first multiplier 1021 input during each data bit. FIG. 3D shows the time (Tb) versus frequency bandwidth (Wss=5*Wb) allocated to the spread spectrum signal in the first multiplier 1021 output during each data bit. Each bit energy is distributed in 5 pieces, each of which is of Wb frequency bandwidth and TcT=b/5 chip time duration. The time and frequency distribution of the band pass filter (BPF) 161 output signal is similar to the time versus frequency bandwidth allocated to the spread spectrum signal in the first multiplier 102 output depicted by FIG. 3D in absence of multiple access interference. The time and frequency distribution (or occupancy) of the low pass filter (LPF) 162 output signal is depicted by FIG. 3E.
In prior art FFHSS techniques, the frequency synthesizer hopping rate is usually considered the major limitation. The FFH encoding/decoding stages require chip rate frequency hopping synthesizers which substantially increases the system cost. Before effective data transmission or reception, the frequency hopping synthesizer (FHS) output is determined (or fixed). During the transmission or reception process the FHS output is a deterministic signal. In the transmission system, only the data signal is random. In principle, only the data rate limits the transmitter minimum rate. However, code generation and frequency synthesizer work at the chip rate.
Spread Spectrum (SS) systems usually require complex signal processing operations, especially in the encoding and decoding steps. In prior art FFH-CDMA, previously used for radio frequency communications, frequency synthesizers at the chip hopping rate are required. The frequency synthesizer hopping rate is usually considered as a major limitation of the system, and substantially increases the system cost. As a result, in Local Area Network (LAN) applications, FFHSS techniques have not been implemented to provide greater sharing of bandwidth among users connected to the LAN. However, there remains a need for providing higher bandwidth shared access to communications media, such as twisted-pair, coax and optical fibers, used in LANs and telecommunications networks.
In the past few years, several CDMA systems using all-optical signal processing systems, including encoders, decoders, power limiters and threshold comparators, have been proposed. Fundamental differences between the optical and the radio communication fields and instruments, such as sources, communication mediums and detection systems, led to the design of some new optical schemes with no parallels in radio COMA systems. Coherent ultra-short pulse sources have been proposed to spectral phase encoding CDMA, however, non coherent broadband sources such as Light Emitting Diodes (LED) and erbium-doped superfluorescent fiber source (SFS) have been proposed for spectral amplitude encoding COMA. These two techniques fall into the so-called frequency encoded (FE) CDMA and have no parallels in radio CDMA. These techniques inherently use very wide bandwidth in the channel, however, they are not considered as SS techniques because the spreading operation is not effectively achieved.
For local area networks with a bit rate on the order of Gigabits per second, optical frequency synthesizers with a chip hopping rate on the order of a tenth of a Gigabit per second are required for optical implementation of the FFH-CDMA technique. However, a practical optical frequency synthesizer has a very limited hopping rate. Slow frequency hopping CDMA (SFH, i.e. one frequency-hop per data bit); and very slow frequency hopping CDMA(one hop per packet of bits) have been previously proposed for optical inter-satellite CDMA communications. The bit rate was limited to a few tenths of Megabits/sec. Furthermore, for local area networks with a bit rate on the order of Gigabits per second, an optical frequency synthesizer with a chip hopping rate on the order of a tenth of Gigabits per second is required for optical implementation of the FFH-CDMA technique.
It is an object of the present invention to provide an FFHSS technique that can be performed with all-optical devices for faster operation than electronic processing methods.
It is another object of the present invention to reduce the processing rate in the transmitter and the receiver. This especially allows fiber optical implementation of the FFHSS technique.
According to a further object of the invention, the processing rate in all of the transmitter and the receiver parts can reduced to the data bit rate, thus avoiding the chip rate frequency hopping synthesis.
An object of the present invention is to provide an FFHSS-CDMA communication system which exploits some deterministic aspects in the signal processing operations to avoid the utilization of real-time chip rate frequency hopping synthesis of the spread spectrum signal in the transmission and in the reception ends. According to the present invention, since the frequency synthesizer output signal is a deterministic periodic signal, passive or lower rate devices can be used in the encoding and decoding stages to avoid real time chip rate frequency hopping synthesis.
Another object of the present invention is to provide a transmitter in a FFHSS communications system which avoids the real time chip rate frequency hopping synthesis.
Another object of the present invention is to provide a receiver in a FFHSS communications which avoids the real time chip rate frequency hopping synthesis.
Another object of the present invention is to provide an embodiment for optical FFHSS system in CDMA local area network architecture.
Yet another object of the present invention is to provide an embodiment for mobile radio frequency FFHSS system which does not require a frequency hopping synthesizer.
According to a first broad aspect of the present invention, there is provided a method of optical signal transmission comprising the steps of generating a multi-wavelength optical signal modulated to encode data and occupy a predetermined fraction of a bit time slot, selecting a plurality of wavelength division slots within a wavelength range of the multiwave-length signal, introducing, according to a code, a predetermined time delay in spectral components of the multi-wavelength optical signal corresponding to each of the plurality of wavelength division slots to displace the spectral components within the bit time slot; and feeding the spectral components delayed according to the code into a waveguide transmission medium shared by at least one other transmitter using the wavelength division slots and a different code.
Preferably, the step of introducing the predetermined time delay may comprise providing an in-waveguide Bragg grating device having a plurality of spaced Bragg grating reflectors for reflecting the spectral component time delayed according to the code. Also preferably, the step of introducing may further comprise providing an optical circulator, coupling the optical signal to a first port of the circulator, coupling the in-waveguide Bragg grating device to a second port of the circulator, and coupling a third port of the circulator to the waveguide transmission medium. The in-waveguide Bragg grating device may comprise an in-fiber Bragg grating, and may also be programmable. The programmable Bragg grating may be adjusted using tensioning devices, such as piezoelectric devices, or using temperature control devices.
Preferably, the code may utilize fewer than all of the wavelength division slots and a bit time slot shorter than a bit time slot used when all of the wavelength division slot is utilized, whereby a shorter code length may be used to achieve a higher bit rate.
According to another broad aspect of the invention, there is provided a method of optical communication comprising the steps of generating a multi-wavelength optical signal modulated to encode data and occupy a predetermined fraction of a bit time slot at a transmitter end; selecting a plurality of wavelength division slots within a wavelength range of the multi-wavelength signal; introducing, according to a code, a predetermined time delay in spectral components of the multi-wavelength optical signal corresponding to each of the plurality of wavelength division slots to displace the spectral components within the bit time slot; feeding the spectral components delayed according to the code into a waveguide transmission medium shared by at least one other transmitter using the wavelength division slots and a different code; receiving the optical signal from the transmission medium; and detecting the displaced spectral components according to the code to recover the data.
Preferably, the step of detecting may comprise: introducing, according to a reverse code complementary to the code, a predetermined time delay in spectral components of the multi-wavelength optical signal corresponding to each of the plurality of wavelength division slots to displace the spectral components within the bit time slot; and detecting only within the predetermined fraction of the bit time slot signal energy of the received optical signal.
Also preferably, the step of receiving may comprise compensating for chromatic dispersion caused by the transmission medium.
When the transmitter end is subject to temperature variations affecting a wavelength of the spectral components, the step of detecting may comprise providing a programmable in-waveguide Bragg grating device having a plurality of tunable spaced Bragg grating reflectors for reflecting the spectral component time delayed according to the code, and tuning the Bragg grating reflectors to compensate for the temperature variations. The tuning of the Bragg grating reflectors may comprise adjusting a temperature control of a temperature control device for each of the Bragg grating reflectors. The tuning of the Bragg grating reflectors may comprise adjusting a voltage control of a piezoelectric element for each of the Bragg grating reflectors.
According to another preferred feature, the code utilizes fewer than all of the wavelength division slots and a bit time slot shorter than a bit time slot used when all of the wavelength division slot is utilized, whereby a shorter code length may be used to achieve a higher bit rate, the step of detecting including steps of: detecting any signal present in at least one unused ones of the wavelength division slots at predetermined time delays; and subtracting the signal detected in the previous step from the displaced spectral components according to the code in order to recover the data.
The invention also provides a method of fast frequency hopping spread spectrum communication comprising the steps of: generating a multi-frequency source signal occupying a wide frequency band; modulating the source signal to encode data and occupy a predetermined fraction of a bit time slot at a transmitter end; selecting a plurality of frequency division slots within the wide frequency band; introducing, according to a code, a predetermined time delay in spectral components of the modulated source signal corresponding to each of the plurality of frequency division slots to displace the spectral components within the bit time slot; transmitting the spectral components delayed according to the code over a medium shared by at least one other transmitter using the wavelength division slots and a different code; receiving the transmitted spectral component from the transmission medium; and detecting the temporally displaced spectral components according to the code to recover the data.
The preferred embodiment of is invention uses band-pass filtering tools and is particularly suitable for optical FFHSS system. In the transmitter, the information bit sequence modulates a broadband source so that the energy assigned to a data bit is concentrated on just a short interval from the bit period (Tb), This interval is in principle less than or equal to Tb/M. In the remaining interval of time, no energy is transmitted. The modulation technique can be frequency shift keying (FSK), phase shift keying (PSK), amplitude shift keying (ASK) or the like. The data modulated signal enters to an equipment which simultaneously or sequentially performs the following three functions generating a signal in an FFHSS form. The first function is a spectral slicing of the input signal leading to a number of sub-pulses, each of which is supported by one different frequency Interval. The second function is a sub-pulse modulation. This modulation can be In ASK, PSK or the like, as prescribed by the code. The third function is a sub-pulse delaying, where each sub-pulse is differently delayed as prescribed by the code frequency hop pattern. The order of these functions depends on the used devices. Some optical devices are proposed in the following description to simultaneously perform the three functions. The final output signal is composed from M sub-pulses, each of which is supported by different frequency bandwidth and positioned in time as prescribed by the code frequency hopping pattern. In the receiver, the received signal is fed to an equipment configured to receive an intended desired user signal. The equipment performs simultaneously or sequentially three functions. The first function (spectral slicing) is similar to that in the transmitter. The second function is the sub-pulse demodulation and depends on the used modulation technique in the transmitter. The third function is also a sub-pulse delaying using the reverse order than the transmitter.